"Hunting," or small random excursions of the rotor in a synchronous motor, is a long known phenomenon. While driving a constant torque load, the rotor motion may have superimposed on its steady synchronous speed a meandering of its rotor phase angle about that of the constant rotating field vector. This excursion is usually oscillatory with a characteristic period, but may have an amplitude and phase which vary randomly. Small synchronous motors which are used in timing and recording devices often display this behavior. The consequent time displacement error may be objectionable or even intolerable in certain precision applications, notably in tape drive motors for video tape recorders.
In the prior art, changes in motor speed have been detected by a tachometer attached to the motor shaft. The output of the tachometer is then fed back through suitable electronic devices to alter the motor driving signals in a sense to reduce the hunting. Another approach is to feed a voltage proportional to the motor oscillations to an eddy current torque brake which acts to apply a varying amount of torque to the motor to reduce the hunting. Each of these prior art systems suffers from a response lag since a mechanical action must take place either to determine the presence of the hunting or to reduce it. This slows down the rate of recovery which may, in critical operations, be unacceptable. Further, in this approach the additional equipment required to sense very small velocity changes and to develope the damping torque is in itself complex and expensive.